Well there are numerous reasons. Maybe you want one business and one personal account and not to mix the two. Maybe you might want to sell or talk/promote something that is considered controversial by some or maybe you want to discuss politics etc without it effecting the way people perceive or do business with you here.
Hm, sounds like a quite good reason. Good point.
But you agree that there is no need to have more than 5 accounts for sure? The persons who participate in the bounty campaign with multi accounts, they most likely have more than 5 accounts, and they are posting from all 5 thus increasing the spam level on the whole forum. I would say that technically possible to create limitations based on MAC/Hardware address, for instance, there are 2 accounts allowed per one device. The business people will be happy enough and bounty-cheaters will be down.